OS X (10.7 or higher): The popular iOS Twitter app (and our favorite) Tweetbot released a public alpha version for OS X. Much like its mobile counterpart, the app feels like a supercharged version of the official Twitter client, adding a number of desired features while maintaining a simple and compact user interface.

One of the most notable features, as Matthew Braga points out, is trackpad gestures. Depending on how your multitouch trackpad is set up, you can swipe with your fingers to move from a tweet to conversation view. Tweetbot also makes it easy to switch between a ton of different views, show multiple timelines, easily view and upload photos, and customize the way it helps you tweet faster (e.g. link shortening, retweeting style, etc.).

It's important to note that this version of Tweetbot is a public alpha release, meaning it's bugging and not feature-complete. It's still missing all sorts of things, there will be graphic glitches, and you're not going to get any support. That said, it's free to try out for the meantime. Tweetbot's developers have noted that the app will cost money when it is finished, but you can use it during the development phase for free. For more information on the alpha release, check out the official announcement.